Multi-stage centrifugal pump: insufficient flow under pressure buildup
Thread Content
The company has a multi-stage centrifugal pump with a capacity of 150 m3 and a head of 300 meters, which is used to supply water to the boiler’s water wall; after passing through the heat exchanger, the water returns to the tank. To maintain the pressure inside the water-cooling wall tubes, a pressure relief valve, also known as a pressure maintaining valve, is installed in the return pipeline between the heat exchanger outlet and the water tank. This valve helps to keep the pressure before it around 3 Mpa, while the pressure after the valve can be reduced to 2–3 kilograms per square centimeter. However, there is a problem: although the pressure relief valve helps to maintain the pressure before it, the flow rate decreases significantly due to the need to hold back that pressure. When the variable-frequency pump operates at 45 Hz, the pump heats up and cannot operate at full capacity. By reducing the pressure before the pressure relief valve to around 2 Mpa, the flow rate increases again, and the pump no longer overheats. I would like to ask: if we want to maintain a pressure of 3 Mpa before the valve, how should the flow rate be controlled to ensure that it remains at 150 cubic meters per hour? There are no bypasses in the entire system.
The pump’s head is 300 m, with a density of 1000; the pressure difference generated by the pump is 3 MPa. The operating point of the pump is also determined by the device’s characteristic curve. What is the inlet pressure of the pump? It’s 3 MPa before the valve, plus the pipeline losses. The pressure difference generated at the pump’s rated point is less than Mpa. One increases the inlet pressure, while the other increases the head.
Why must the pressure before the control valve be 3 MPa? Can’t the pressure be lower?
To maintain a pressure of 3 MPA, a pump flow rate of 150 cubic meters per hour is required; this can be determined by referring to the pump’s performance curves from its factory tests. It is recommended to install pressure gauges on pipelines with the same diameter at both the inlet and outlet (the inlet can be at atmospheric pressure, or a pressure gauge can be installed only at the outlet). When the pressure difference between the inlet and outlet is 3 MPA, it can be assumed that the pump’s flow rate is 150 cubic meters per hour (at its rated speed). However, if the pump still heats up or exhibits abnormal behavior under these conditions, then it relates to the pump’s reliability, rather than its performance parameters
The pump’s rated flow rate and pressure are 150 cubic meters per hour and 3 MPa respectively. It is the back pressure valve that prevents an adequate pressure from being maintained, resulting in insufficient flow at the pump’s outlet. At an outlet pressure of 3 MPa, no flow occurs – only a very small amount of fluid flows out. If the back pressure valve is removed, everything functions normally; without it, however, no pressure can be maintained.
Our problem now is that we need a pressure holding valve to maintain the pressure of the entire system at 3 Mpa, while also ensuring an adequate flow rate. I feel these two requirements are somewhat contradictory; I’m not sure how to resolve them
As mentioned above, you can increase the pump’s head pressure
The head is 300 meters. How can it be increased further? It’s not asking us to replace the motor, is it?
Does changing the motor speed increase it? Unless it can be frequency-transformed to 60 Hz. With the pressure required by your system plus the pipe losses, how can the pump still deliver flow?
Buy the pump first, and add the pressure regulator later? If the problem is to be solved simply by working on the pump, as mentioned above, it is necessary to recalculate the head after installing the valve and use a pump with a higher head. Or you can look at the pump’s performance curve and find a compromise between head and flow rate to see if it will work
Can’t the backpressure valve pressure be adjusted? Can’t it be turned down a bit?